This invention relates in general to ion impact surface modification techniques and, more specifically, to methods and apparatus for improving the physical characteristics of the internal walls of tubes by ion impact treatments including ion implantation, ion mixing, and ion beam assisted deposition of coatings.
Piping in power plants and the like often suffers from corrosion and erosion. These problems require regular inspection and replacement, which often requires the plant to close down for repair and replacement work. Improvements in interior surface hardness and improved corrosion resistance would reduce the need for such costly repairs.
Corrosion in nuclear reactor piping systems is a particularly significant problem. Corrosion products from the pipes and other system components flow into the reactor and become radioactive. These then settle in low points in the system, typically valves, drains and pumps, representing a radiation hazard to personnel doing maintenance on the reactor. Corrosion products circulating in reactor piping also adversely affects the operation of flow venturies that are used to measure flow. The venturies are restrictions that permit water flow to be calculated from the measured pressure drop across the restrictions. Corrosion products selectively deposit in these venturies, altering the flow calibration such that the reactor power level must be reduced by as much as a few per cent, at significant cost to the plant operator. Various additives are used to reduce corrosion and expensive, more corrosion resistant, metals must often be used. Thus, there would be significant savings in any pipe treatment that reduced corrosion.
Erosion of the interior of pipes subjected to high flow rates is a problem in many piping systems. Reduction in erosion could significantly increase the life span of such piping components. Uniform hardening of the interior surfaces could significantly reduce erosion damage.
High vacuums are required in various specialized tubular systems, such as high energy physics experiments (such as the Superconducting Super Collider) and fusion energy systems. In order to maintain high vacuums in such systems, outgassing through the component walls must be reduced or eliminated. Presently, no full effective system for preventing such outgassing exists.
Effective treatment or coating of the interior surfaces of such tubes or tubular components could be very advantageous in reducing corrosion, erosion and outgassing.
A number of different methods have been developed for depositing materials, generally metals, in the form of particles or ions onto a target surface to form an adherent, uniform coating. Among these are thermal deposition, cathode sputtering and chemical vapor deposition. While useful in particular applications, these methods suffer from several problems, including a tendency to coat other system surfaces than the target with the material being deposited, requiring frequent cleaning, contamination problems when the coating material is changed and a waste of often expensive coating material. Generally, these processes require that the target surface be heated to a very high temperature which often damages the target material. The high deposition temperatures also lead to thermal stresses that may cause coating delamination. These processes are quite effective in coating flat or slightly curved surfaces, but are not adaptable to coating the interior surface of relatively narrow tubes. Where not very highly adherent, these coatings may not effectively harden or change the surface to increase resistance to corrosion and erosion, and generally are too porous to prevent outgassing.
Vacuum arc deposition has a number of advantages for coating difficult materials, such as refractory metals, onto targets. Vacuum arc deposition involves establishing an arc, in a vacuum, between a cathode formed from the coating material and an anode, which results in the production of a plasma of the cathode material suitable for coating. The process does not involve gases, making control of deposition rate easier and simplifies changing coating materials. Typical vacuum arc deposition systems are described in U.S. Pat. Nos. 3,566,185, 3,836,451 and 4,714,860. Vacuum arc deposition, sometimes referred to as cathodic arc deposition, is used commercially, typically to produce titanium nitride coatings on tooling. While the coatings formed by these methods are generally hard and resistant to erosion, they may not resist corrosion or erosion to the full extent required. Vacuum arc deposition is a line-of-sight process, making it virtually impossible to modify or coat the inner surfaces of tubes and pipes with existing technology.
Thus, there is a continuing need for methods and apparatus for treating and/or coating the interior surfaces of pipes and tubes to form a corrosion, erosion and outgassing resistant surface.